Figure 7. An overlay of 60-ms records shows the relation of FV amplitude to excitatory postsynaptic potential (EPSP) amplitude with increasing stimulus intensities (A). The lower case letters shown are for different stimulus intensities and correspond to the time points indicated in B. Stimulus intensity ranged from 1.5 to 5.0 V. Time versus EPSP amplitude is described by the solid line and the left axis (EPSP); time versus FV amplitude of the same preparation is shown by the dotted line and the right axis (FV). At 32 min, the stimulus intensity was reduced in steps and then gradually increased back to initial strengths (42 to 60 min). Excitatory postsynaptic potential amplitude versus corresponding fiber volley amplitude with increasing stimulus intensity is shown in C, both values expressed as a percentage of maximum response. A best fit to the data was found to have a slope of 1.2 over the linear region. A 20% decrease in FV amplitude would result in a 24% decrease in EPSP amplitude.